The correct answer is, “Barack Obama is not on the ballot in 2016 ... is he? Let’s stick to something more relevant.”
It's apparent that Gov Walker, so far, is coming prepared for this onslaught and is proving to be ready for prime time. I would've prefaced your comment with something very similar to the response he did give. Along the lines of, "The president (small 'p' in Obama's case) & I have never had a discussion on theology. Therefore, I don't know what's in his heart. Perhaps you should ask him!".